This invention relates to a ball for a ball-point pen.
A ball for a ball-point pen is held rotatably in a ball holder tip of the ball-point pen, with part of the ball protruding outside the ball holder tip. The ball touches a writing surface, which is usually paper, and serves to transfer ink from an ink supply to the writing surface. This is shown, for example in FIG. 8 of Japanese Patent Application Publication No. JP 2000-168286 A. The term xe2x80x9cPatent Application Publicationxe2x80x9d is hereinafter referred to as xe2x80x9cPub.xe2x80x9d. The ball generally has a nominal diameter in the range from 0.3 to 1.2 mm. The ball is usually made of a sintered hard material such as a cemented carbide or ceramic.
In the process of writing, the ball rotates while pressure is applied through the ball to the writing surface. Friction occurs at the area of contact between the ball and a conical ball seat inside the ball holder. This friction abrades the ball seat, and, as a result, the ball tends to sink into the ball holder, as shown in FIG. 13 of the above-mentioned published Japanese application. This phenomenon, which is called xe2x80x9cball dropxe2x80x9d or xe2x80x9cball downxe2x80x9d by ball-point pen manufacturers, gives rise to several problems. First, smooth rotation of the ball is obstructed, which gives a feel to a writer as if the ball is scratching the writing surface. Second, the ink discharge rate changes from its initial setting, and xe2x80x9cblurxe2x80x9d or xe2x80x9cline-breakagexe2x80x9d occurs in written traces. In addition, the ink has a tendency to flow out suddenly, producing large quantities of ink as a blot or smear, which is called xe2x80x9cgoopxe2x80x9d. Third, when the contact area in the ball seat is abraded excessively, writing may become impossible. In short, abrasion of the ball seat results in a deterioration in writing performance.
Accordingly, it is desirable that the ball be less prone to abrade the ball seat. The term xe2x80x9cwriting performancexe2x80x9d as used herein in ball evaluation, refers to writing performance from the viewpoint of ball seat abrasion.
In a ball-point pen, there is also a requirement that ink consumption be kept below a standard value for a predetermined writing distance. If the wetting relationship between the ink and the ball, which is hereinafter referred to as xe2x80x9cspreadabilityxe2x80x9d of the ink, is too high, the ink discharge becomes excessive, and hence the ink consumption exceeds the standard value.
In addition, there are two opposed problems. If the spreadability of the ink is too high, that is, the ink discharge is excessive, xe2x80x9cgoopxe2x80x9d is prone to occur in the written traces. On the other hand, if the spreadability of the ink is too low, that is, the ink discharge is insufficient, xe2x80x9cblurxe2x80x9d, xe2x80x9cline-breakage,xe2x80x9d or xe2x80x9cshadexe2x80x9d occur in the written traces. In other words, for proper writing performance, the spreadability of the ink must be within a suitable range.
To address the aforementioned problems, the following balls have been proposed: (1) A ball which has an arithmetic average roughness (Ra) of 5-15 nm, as disclosed in Pub. No. JP 10-250280 A(1998); (2) A ball which has both an arithmetic average roughness (Ra) of 4-10 nm and a maximum height (Ry) of 150 nm or less, as disclosed in Pub. No. JP 10-329473 A(1998); and (3) A ball which has both the arithmetic average roughness (Ra) of 5-25 nm in at least a 50 xcexcmxc3x9750xcexcm region on the surface and a maximum height difference between peaks and valleys (Pxe2x88x92V) of 150-250 nm, as disclosed in Pub. No. JP 2002-103871 A.
However, as shown in Table 1 below, which tabulates the surface roughness of balls made of a cemented carbide and having a nominal diameter of 1 mm, and the evaluation for respective balls performed by a certain ball-point pen manufacturer, I have found that the foregoing problems cannot be solved only by controlling the average roughness (Ra) of the balls. The symbols xe2x80x9c∘xe2x80x9d and xe2x80x9cXxe2x80x9d in Table 1 denote xe2x80x9cacceptablexe2x80x9d and xe2x80x9cunacceptablexe2x80x9d respectively.
In Table 1, the arithmetic average roughness (Ra) of product #1 is the largest among the five products. Therefore, it was expected that the spreadability of the ink would be too high, i.e., the ink discharge would be excessive, and hence the rating for ink consumption would be xe2x80x9cXxe2x80x9d. However, the rating was xe2x80x9c∘xe2x80x9d, contrary to the initial expectation. The arithmetic average roughness (Ra) of product #5 was within the range of the proposed values or close to the upper limit values. However, the rating for ink consumption was xe2x80x9cXxe2x80x9d. In addition, the maximum height (Ry) and the maximum height difference between peaks and valleys (Pxe2x88x92V) are also parameters, each indicating only a profile variation in a longitudinal sectional direction on the ball surface, just as the arithmetic mean roughness (Ra) does. Hence the use of these two parameters cannot solve the foregoing problems.
For the tests tabulated in Table 1, the arithmetic average roughness (Ra) was obtained as follows. Five balls were sampled at random from each product lot. With a surface texture measuring instrument capable of measuring three-dimensional roughness in a non-contact manner (e.g., xe2x80x9cForm Talysurf PGIxe2x80x9d produced by Taylor Hobson Ltd.), the surface roughness was measured in one arbitrary location on the surface of each sampled ball, namely, inside a circular region thereon having a diameter of 0.1 mm, which is 10% of the 1 mm nominal diameter of the ball. Then, the cut-off value was set to 0.11149 mm, and the arithmetic average roughness (Ra), which is a three-dimensional parameter, was obtained with this cut-off value.
Insofar as writing performance is concerned, a ball is rated xe2x80x9c∘xe2x80x9d if the ball gives no feel of scratching the writing surface and a smooth feel in writing can be obtained, and it does not produce xe2x80x9cblurxe2x80x9d, xe2x80x9cline-breakagexe2x80x9d, xe2x80x9cgoopxe2x80x9d and xe2x80x9cshadexe2x80x9d in the written traces. On the other hand, if the ball gives a feel of scratching the writing surface, and a smooth feel in writing cannot be obtained, or if ball produces any one of the following phenomena, namely xe2x80x9cblurxe2x80x9d, xe2x80x9cline-breakagexe2x80x9d, xe2x80x9cgoopxe2x80x9d and xe2x80x9cshadexe2x80x9d in the written traces, the ball is rated xe2x80x9cXxe2x80x9d.
Insofar as ink consumption is concerned, the spreadability of the ink is suitable, that is, the ink discharge is suitable, and consequently the ball is rated xe2x80x9c∘xe2x80x9d, if the ink consumption is below a standard value when the ball is caused to travel against a writing surface over a predetermined writing distance. On the other hand, the spreadability of the ink is too high, that is, the ink discharge is excessive, and the ball is rated xe2x80x9cXxe2x80x9d, if the ink consumption exceeds the standard value when the ball is caused to travel against the writing surface over the predetermined writing distance.
I have carried out research on the properties of the ball carefully taking note of skewness (Rsk), kurtosis (Rku) and the average spacing of local peaks (S), in addition to the arithmetic average roughness (Ra), which is one of the roughness parameters. As a result of my research, I have determined the following. First, when the cut-off value is specified (specifically, when the value is set to 0.00132 mm, as discussed below), the product of the skewness (Rsk) value and the kurtosis (Rku) value, which are three-dimensional parameters, namely, the value of Rskxc3x97Rku, correlates with writing performance. Second, the ratio of the arithmetic average roughness (Ra) value, which is a three-dimensional parameter, to the average spacing of local peaks (S), which is a two-dimensional parameter, namely, the value of Ra/S, correlates with the ink consumption (i.e., the ink discharge). These findings have led me to this invention.
An object of this invention is to provide a ball for a ball-point pen, which is less prone to abrade a ball seat inside a ball holder of the ball-point pen, and hence is capable of maintaining a pen""s initially excellent writing performance over time. Another object of the invention is to provide a ball for a ball-point pen which exhibits suitable spreadability of the ink, and hence can keep ink consumption below a standard value when the ball is caused to travel over a writing surface through a predetermined writing distance.
In order to attain the above objects, in accordance with one aspect of the invention, the ball is made to satisfy the following relations: xe2x88x922.5 less than Rskxc3x97Rku less than xe2x88x921.2; and 0.005 less than Ra/S less than 0.012, where respective values of Ra or arithmetic average roughness; Rsk or skewness; Rku or kurtosis which are three-dimensional parameters; and S or average spacing of local peaks, which is a two-dimensional parameter, are obtained with a cut-off value set to 0.00132 mm, when the surface roughness of the ball is represented by a value in one arbitrary location on the ball surface, namely, inside a circular region thereon having a diameter which is 10% of the nominal ball diameter. The ball structured as described above exhibits the following advantages.
With Rsk and Rku obtained with a cut-off value set to 0.00132 mm, and the product of the skewness (Rsk) value and the kurtosis (Rku) value satisfying the relation xe2x88x922.5 less than Rskxc3x97Rku less than xe2x88x921,2, the ball becomes less prone to abrade the ball seat inside the ball holder, and hence the ball is capable of maintaining the initial excellent writing performance of the ball-point pen.
With Ra and S obtained with a cut-off value set to 0.00132 mm, and the ratio of the arithmetic mean roughness Ra to the average spacing of local peaks S satisfying the relation of 0.005 less than Ra/S less than 0.012, the spreadability of the ink, i.e., the ink discharge, becomes suitable, and hence the ink consumption can be kept below the standard value when the ball is caused to travel over the predetermined writing distance. Furthermore, the writing performance of the ball from the viewpoint of the spreadability of the ink is improved.